Philips PM 5775 Manual page 30

If switch CONT. GATED, S6, is in position CONT. the —20 V supply is switched
off, so that Z203 conducts and Z204 is cut off.
. Astable multivibrator
The oscillator is an emitter-coupled astable multivibrator consisting of transistors
Z206 and Z208. The time determining elements R and C are represented by R211,
R213, RV202, RV203, RV1A, C209 and C1...C9. Capacitors C1...C9 are connected
in parallel with C209.
Via zener diode Z205 and resistors R212 and R284 a bias voltage of —5 V is fed
to the base of Z206.
The currents through Z206 and Z208 are supplied by transistors Z207 and Z259
which may be regarded as constant current sources. The bases of Z207 and Z259
are biased to about —10 V, whereas the total current is adjustable by means of
RVIB.
The frequency setting of the multivibrator is effected by switch REPETITION
TIME, S1, and potentiometer RV1A. This potentiometer is incorporated in the
collector circuit of Z206. This means that the amplitude of the multivibrator
signal depends on the frequency. This amplitude variation, however, is reduced
by potentiometer RV1B which is coupled with RV1A. When the frequency of the
multivibrator is e.g. increased by a decrease of RV1A this will result in a smaller
amplitude. However, the current through Z207 and Z259 will increase because the
setting of RV1B is varied at the same time. The degree of this current increase is
such that the amplitude variation will be eliminated.
. Gate function
External triggering and single shot operation.
Diodes Z212 and Z213 form an AND-gate, i.e. both cathodes should be positive to
affect Z215.
In position TRIGG. or SINGLE SHOT of switch REPETITION TIME, S1,
transistor Z208 is cut off because its emitter is connected to earth via switch S1.
The cut-off-stage of Z208 implies a high level on the cathode of Z213 so the latter
will cut off.
Then the differential amplifier Z215-Z218 can be switched over only if the voltage
level on the cathode of Z212 becomes high. This voltage level becomes high by
either supplying trigger pulses to socket TRIGG./GATE IN, P1, or depressing
SINGLE SHOT switch SS.
Internal operation, externally gated
In this operation mode, the REPETITION TIME switch S1 occupies one of the
time positions between 10 ns and 100 ms and switch CONT. GATED S6 is set to
position GATED. The multivibrator is inoperative due to the high collector level
of Z203.